The attraction between [C2HJ2and [C2HJ23and Tl(I) in the hypothetical [Tl(C2H4)2]+ and [Tl(C2H2)n]+ (n = 2,3) complexes was studied using ab initio methods. It was found that the changes around the equilibrium C-Tl distance and in the interaction energies are sensitive to the electron correlation potential. We evaluated these effects using several levels of theory, including HF, MP2, MP4 and CCSD(T). The equilibrium C-Tl distances in the complexes are in the range of 294 pm to 338 pm. The interaction energy differences obtained at the equilibrium distance range from 55 to 110 kJ/mol at the different levels used. These results indicate that the complexes formed are in the category of van der Waals systems. Also, these results indicated that the interaction between olefinic systems and Tl(I) are a real minimum on the potential energy surfaces.